This invention provides a source of metal halide gases for large scale CVD reactor systems that enable the deposition of thick, low stress, coatings on metallic substrates that are of high purity, crack free, and adherent.
The invention provides a long lasting source of halides and, since it needs refilling at rare intervals, lessens the possibility of process variations or contamination.
The metal halide generation is controllable and self limiting and the deposition reaction within the CVD furnace has low drift and easy flow control.
The construction of the metal halide source vessel and outer housing insures maintainable high vacuum seals and has heaters protected from corrosive influences and contamination.
This invention further provides for the use of a variety of metal forms usable as source materials subject only to the requirement that gas flow can be maintained through the metallic source materials.
Metal halides, especially the chlorides, are critical components of successful chemical vapor deposition (CVD) of pure metal layers in a low pressure reactor/furnace due to their reactivity and the ease of deposits that are pure. The atom of the halogens is large, easily react in the deposition process and in many cases have no interaction in a CVD plasma assisted environment with the substrate being coated. The size of the molecule and mode halide dissociation in the reaction also contributes to the deposited metal being exceptionally halide free.
Supply of a metal halide gas to a pilot or research reactor is relatively easy due to the small scale encountered in such units. Knudsen cells with a metal compound containing crucible surrounded by a heater inserted into a port of the reactor vessel for example provide small quantities of metal halide or other gases that are ejected (effused) from the cell into the reactor. These cells are for one use and their changing can create contamination. Gaseous metal halides may be available in small quantities for direct injection into small reactors. For commercial size reactors gas supply and prevention of contamination are critical limiting factors. The present invention provides a long lasting, multiple use source for metal compound formation and gasification through self limiting reactions to provide contaminant free reactive metal halide vapor feed to large commercial CVD furnaces.
The use of chromium in chromizing extends back to at least the early fifties. The National Bureau of Standards had translated Russian books on the subject in the 60's where chrome was made available by the packing of parts in a chrome or chrome ferroalloy powder with enough fillers (clays) to prevent surface welding. The chrome packed parts were typically welded into a container which was then heated as hydrochloric acid fumes were introduced. The halide is constantly recycled in formation of metal halides and then deposition of the metal. The reaction is not selective and ferrocompounds form and thus purity cannot be achieved. A number of hours at over 1000 degrees C. forced the chrome as metal atoms into the surface of the parts with a net surface content of up to 85% chrome by weight and diffusion into the base metal. At the end of the heating, the container is cut apart and parts and filler are separated. This was the state of the art for a number of years until better technology was developed.
With the advent of sputtering, where ions were accelerated by electromagnetic force to impinge on a surface higher purity metal deposits (especially chrome) were possible. This method however was limited to line of sight deposition from the emitter part of the sputtering apparatus, and large parts were difficult due to the dispersal pattern of the metal molecules.
CVD, the chemical vapor deposition in the late 80's provided another method for the deposition of metals, this time free of the line of sight restriction. As the technology advanced the possible products went from complex thin layer metal coatings such as used on microchips. The use of titanium and other low boiling point metal halides increased and TiN and TiCN became popular. The plasma enhanced CVD (PECVD) furthered the progression of the strong electrical attraction to a target part plus chemical attraction now possible. The stage was set for development of thicker pure metal deposits on parts and this was possible with the low reactivity easily vaporized compounds of metals the highly reactive and high temperature reactive metals such as chromium were not as easily handled and the limitations of precursor metal halide supply prevented the commercial deposition of thick metal layers of many desired metals.
Metal halides are an excellent source of reactant for CVD processes due to lack of secondary reactions with the halogen and the reactivity caused by temperature and pressure dependent decomposition of the metal halide molecule. A key problem is that metal halides are a nightmare to handle. While there are metal chlorides that are liquid at room temperatures (TiCI4 for example), stable or relatively stable at these temperatures despite being highly corrosive, other halides are highly corrosive and exist as a liquid or gas only at high temperatures. Minor contaminants may also cause deposition of products or much accelerated corrosion. The first group (room temp liquids and moderately reactive) are used in many common processes such as formation of titanium nitrides or other titanium compounds. The second group has allowed experimental development work by use of small halide injectors or vaporization cells of various types. The problem is the scale up of these lab sized injectors into a commercial scale where the corrosion and related contamination problems have stymied process development. The result has been the use of other compounds such as organic chrome compounds or different and less useful processes such as sputtering instead of the more difficult ways of generating and using metal halides with CVD or PACVD processes (PACVD and PECVD are interchangeable in the context of this disclosure).
To form a gas of a metal halide, high temperatures are required for a major portion of the possible metal halide formation processes and the high temperature halide portion can react with and remove metal ions or atoms from such normally non-reactive materials as Hastelloy alloys, pure nickel, and many other normally non-reactive substances and metals. Even if the goal of the CVD deposition is a metal alloy, reactions that occur uncontrolled in the metal halide delivery system can cause an imbalanced alloy or cause improper alloys to form. When, in the more usual case pure metals are desired, contamination creates fatal flaws.
One good example of the corrosion problem was a hastelloy tube where the hot chromium chloride gas corroded both ends, but had relatively little effect on the center portion. Such temperature biased corrosion means that as conditions change even slightly, the zone of corrosion and the extent and content of corrosion products would change resulting in a process totally out of control. A totally non-reactive container is needed for the reactor that remains non-reactive from the first heating of halogen containing incoming gases to the actual delivery into a reactor and the reaction zone therein.
The non-reactive container must still be heated, must be supported within a vacuum capable chamber or feed element, and be removable as needed without damage to the vacuum containing capability of the production system. There is a lack of devices acting continuously that provide all of these needed elements.